1. Field of the Invention
The present invention relates to a digital communication system for use in networks such as U.S. SONET (Synchronous Optical Network).
2. Related Art
Conventionally, networks of this type have been adapted to use a frame having a format known as STS-1 as shown in FIG. 5 to transmit information of 90 bytes.times.9 rows per frame at a rate of 51.84 Mb/s. In the frame of FIG. 5, the bits represented by (a), (b) and (c) are known as overhead bits into which control data for controlling the network is inserted, and the bits represented by (d1) and (d2) indicate communication data (information to be transmitted).
With conventional digital communication systems for use in such a network when data is transmitted between first and second stations, the second station switches the clock thereof in response to an alarm received from the first station, i.e., LOS (loss of signal), LOF (loss of frame) or LAIS (line alarm indication signal). In this case, however, it is difficult to select an optimum clock according to the deterioration degree of line condition (transmission quality), thereby lowering signal accuracy.
The first station successively transmits to the second station frames each having communication data and parity check data inserted therein based on a first clock generated in the first station. The second station performs parity calculation on the communication data in each of the received frames to judge if the calculation result matches the parity check data or not, then inserts numeric data indicative of a mismatch count into a corresponding frame, and successively transmits the frames each inserted with numeric data to the first station based on a second clock generated in the second station. The first station adds up the numeric data for each unit time to detect line condition between the first and second stations from the addition result (sum of parity errors per unit time). In this case, however, a small or no phase difference between the first and second clocks in the second station often causes the numeric data to fail to be inserted into the corresponding frame or causes the same numeric data to be inserted into two successive frames, resulting in a large error of the sum of the numeric data calculated in the first station.